More than 3,000 women were interviewed for the study. Of those, 1,101 were women living with advanced ovarian cancer, 389 with borderline or early ovarian tumours and 1,832 were control subjects — women without ovarian cancer.

Women treated with radiation for breast cancer are more likely to develop heart problems later, even with the lower doses used today, troubling new research suggests. The risk comes from any amount of radiation, starts five years after treatment and lasts for decades, doctors found.

Patients shouldn’t panic — radiation has improved cancer survival, and that is the top priority, doctors say. The chance of suffering a radiation-induced heart problem is fairly small.

For example, four to five of every 100 women who are 50 years old and free of heart risks will develop a major cardiac problem by age 80, and radiation treatment would add one more case, the research suggests.

A quarter of the women interviewed for the study living with advanced cancer said they had worked night shifts in comparison to a third of the women with borderline tumours and one in five of the women in the control group.

Overall, the study reveals a 24% increase in risk of advanced cancer and a 49% increase in risk of early-stage tumours for women who work night shifts as opposed to their counterparts who worked during the day. Researchers also found that risk did not increase for the women who had worked night shifts the longest.

While these findings support earlier research linking shift work to breast cancer, experts warn that more study is needed in these areas.

Study leader Dr. Parveen Bhatti of the Fred Hutchinson Cancer Research Center in Seattle told BBC News that more research was necessary to fully understand the study’s findings and its implications.

“There were a lot of details about shift work that we weren’t able to capture,” Bhatti said.

Researchers hope to further explore chronotypes and their connection to cancer and shift work

Bhatti told BBC News that researchers hope to further explore chronotypes and their connection to cancer and shift work. During the study researchers identified each woman’s chronotype, which refers to whether she identified herself as a morning person — a “lark” — or as a “night owl.”

Some evidence from the study suggests that women who identified themselves as night owls could be more adaptable to working night shifts, as they were found to have have a lower risk of cancer.

The International Agency for Cancer Research has previously reported that interruptions of the body’s natural “clock” are a probable cause of cancer. The disruption of the sleep hormone melatonin is one explanation for the link between shift work and cancer.

Melatonin, produced at night during normal sleep cycles, is disrupted during night shifts. This disruption can have a domino effect on the body’s production of other hormones and weaken natural defenses against DNA damage.